I am still looking into this one, since no engineer can give me anything valuable yet. Hooked on the DC Bus 1 is the AFCS system "A" F.C.C. and on DC Bus 2, the AFCS system "B" F.C.C.
When the APU Generator is the only source of AC power it supplies both AC transfer busses through the closed BTB's. Since all of the required busses are powered at that time, the only new event on the approach is the opening of the cross bus tie at G/S intercept which separates DC Bus 1 from DC Bus 2. It may very well be, that since there is only one Generator supplying all of the load that the effected F/D are biased out of view because of the lack of redundancy, as Track mentions.
The right ADIRU does lose power momentarily when the loss of both generators occured, but it is not lost per se, since it can stay on DC power for the next 5 minutes. The APU start and the subsequent APU Gen powering both AC transfer busses happens before that time limit, thus the right ADIRU is not lost in this situation.
I shall be climbing back in the box on wednesday and I hope to find out more about this enigma. Stay tuned and keep your coments coming!